It is known to provide turbine rotor blades with a protective layer, in order for them to have an increased lifetime during operation in a gas turbine. Often applied as a protective layer to the turbine rotor blade produced in a casting process is a corrosion protection layer of the type MCrAlY. The application of the protective layer takes place in the region of the surface that is exposed to the hot gas of the gas turbine during operation. This region comprises both the blade airfoil and the platform of the turbine rotor blade, on which the blade airfoil is integrally formed. Apart from the corrosion protection layer, a thermal barrier coating may be applied in the aforementioned region, in order to minimize as much as possible the amount of heat introduced into the base material of the turbine rotor blade from the hot gas. The application of the layers thereby changes the vibrational behavior of the turbine rotor blade.
It is also known that turbine rotor blades are excited to vibrate during the operation of the gas turbine. The vibrational excitation is caused by the rotation of the rotor on which the turbine rotor blades are secured. Also contributing to the vibrational excitation of the blade airfoils of the turbine rotor blades is the hot gas impinging on them. Since the blade airfoils of the turbine rotor blades rotate downstream of a ring of turbine guide vanes—seen in the direction of flow of the hot gas—they are excited to vibrate by hot gas pulsating on them. It is therefore required that each turbine rotor blade has a sufficiently high resonant frequency, so that the respective excitation frequencies of both the vibrational excitation originating from the rotational speed of the rotor and the vibrational excitation originating from the hot gas do not lead to an unacceptably great vibration of the blade airfoil. Accordingly, in the prior art the turbine rotor blades are designed in such a way that their resonant frequency deviates from the excitation frequencies of the stationary gas turbine. In the development of the turbine rotor blade, it is also ensured that, overall, the finished turbine rotor blade satisfies the requirements with respect to natural resonance, including with regard to the rotor speeds to be expected.
It is therefore envisaged in the production process of the turbine rotor blade to test each individual turbine rotor blade for its vibrational properties. In this test, the turbine blade is clamped at the root and made to vibrate by a mechanical impulse. Then the vibrational response of the turbine blade, and in particular its blade airfoil, is sensed. If the vibrational response of the turbine rotor blade does not comply with the predetermined frequency values for the resonant frequency, it must be discarded or manipulated by means of suitable measures in such a way that it meets the requirements for the resonant frequency, and is consequently suitable for operation. In order that turbine rotor blades that are not intended to be used in the gas turbine just because of their vibrational property are nevertheless passed on for use, it is known for example from EP 1 985 803 A1 to introduce a recess in the tip of the blade airfoil, whereby the mass of the turbine rotor blade can be reduced at its free, vibratory end. By reducing the mass of the turbine rotor blade, the vibrational property is positively influenced. Its resonant frequency can be shifted to higher values by removing the mass.
In addition, it is known from EP 0 537 922 A1 to insert a tubular damper in the blade platform of a turbine rotor blade. This damper can be pushed out slightly under centrifugal force, and thus come into contact with a platform of a neighboring blade to dampen blade-to-blade vibrations during operation.